Composition and process for zinc phosphate conversion coating

ABSTRACT

A zinc phosphate-type conversion film having microfine-sized crystals is formed on metal surfaces using a conversion treatment bath that contains zinc ions and phosphate ions along with 50 to 1500 ppm of an organoperoxide conversion accelerator, and optionally surfactant. Surface-conditioning treatments can be omitted from this method. The presence of the surfactant makes possible simultaneous execution of surface cleaning and conversion treatment

DESCRIPTION

1. Field of the Invention

The present invention relates to zinc phosphate-based conversion coatingor treatment compositions for application to metals) for example, steelsand zinc-plated steels, and to methods for the zinc phosphate-basedconversion treatment or coating of metals. More particularly, thisinvention relates to a zinc phosphate-based conversion treatmentcomposition, often hereinafter called a “bath” for brevity, even whenused by some method other than immersion, and method that can uniformlycoat metals with a fine, dense zinc phosphate-type conversion coatingthat contains extremely small conversion crystals and that, based on thepresence of said microfine crystals, can improve the adherence of thezinc phosphate-type conversion film to paint films.

2. Description of Related Art

At present, a zinc phosphate-based conversion treatment is executed as apretreatment on various metals when the metal is to be painted orsubjected to cold working. This pretreatment is carried out in theformer case in order to improve the post-painting corrosion resistanceand the paint film adherence and in the latter case in order to improvelubrication during cold working.

The conversion treatment baths used in zinc phosphate-based conversiontreatments are essentially acidic aqueous solutions that contain zincions, phosphate ions, and oxidizer. Nitrite salts, chlorate salts,hydrogen peroxide, organic nitro compounds, hydroxylamine, and the like,are usually considered for use as the oxidizer. These oxidizers functionto accelerate the conversion reactions and so are generally calledconversion accelerators. While a nitrate salt may be present in theconversion treatment bath, nitrate salts do not exhibit an oxidizingfunction in zinc phosphate-based conversion treatment baths and so aredistinct from conversion accelerators.

In the case of the conversion treatment of ferriferous metals, one roleof the conversion accelerator in zinc phosphate-based conversiontreatment is to oxidize the divalent iron ions eluted into the bath totrivalent iron ions. The conversion reactions are inhibited, forexample, by the accumulation of divalent iron ions during the continuousconversion treatment of ferriferous metals, so the role of theconversion accelerator in preventing accumulation of the divalent ironions is extremely important.

However, the known conversion accelerators are each associated withproblems that must be solved. For example, in the case of the nitritesalts, which are at present the most widely used conversionaccelerators, these are unstable in the acidic region and are thusconsumed by spontaneous decomposition even when no conversion treatmentis being run and the bath is merely stored. This requires continual makeup of the consumed amount in order to maintain a constant concentration.

Furthermore, as is known some of the nitrite salt is converted to NO_(x)during the spontaneous decomposition or the intended oxidation activity,and this NO_(x) diffuses into the atmosphere as a pollutant.

In the case of chlorate salt conversion accelerators, chloride ions areproduced during conversion treatment as a decomposition product andaccumulate in the conversion treatment bath. The corrosion resistance ofthe metal suffers a drastic decline when even a trace amount of thechloride ions in the conversion treatment bath remains present on thesurface of the treated metal. Moreover, although chlorate salts aregenerally used in combination with another conversion accelerator, suchas a nitrite salt, the use of a chlorate salt by itself results in asubstantial reduction in the conversion reaction rate.

The use of hydrogen peroxide as a conversion accelerator is associatedwith problems of stability in the conversion treatment bath, andhydrogen peroxide is readily decomposed by dissolved oxygen in theconversion bath. In addition, hydrogen peroxide has a narrow optimalconcentration range in conversion treatment, which makes management ofthe conversion treatment bath quite difficult. When the dissolvedhydrogen peroxide concentration is too high, a poorly adherentpowder-like conversion film is deposited on the metal surface.

Problems also occur with the use of nitrogenous organic compounds suchas organic nitro compounds (e.g., nitroguanine, sodium meta-nitrobenzenesulfonate, etc.) as a conversion accelerator. For example, in the caseof nitroguanine, this compound has a low water solubility and thuscannot be formulated as a concentrate for addition to the conversiontreatment bath. Moreover, it has a weak oxidizing activity for divalentiron ions and so provides poor control of the divalent iron ionsconcentration in the conversion bath. Sodium meta-nitrobenzene sulfonateby itself has a poor conversion activity and must generally be used incombination with another stronger conversion accelerator. Itsconcentration management also requires large-scale measurementinstrumentation, such as an ion chromatograph. In addition, theaccumulation of these organic nitro compounds and their decompositionproducts in the conversion treatment bath causes an increase in the CODof the conversion treatment effluent, which has a negative effect on theenvironment.

With regard to the use of a hydroxylamine compound as a nitrogenousorganic conversion accelerator, such a compound must, for best results,be added to the conversion treatment bath in concentrations of at least1,000 ppm, which causes a large, uneconomical consumption of theconversion accelerator.

The use of chromic acid and permanganate salts as a conversionaccelerator for zinc phosphate-based conversion treatment baths has beeninvestigated (Norio Sato, et al., Boshoku Gijutsu [English title:Corrosion Engineering], Volume 15, No. 5 (1966)). These authors reportedthat the formation of conversion coatings was not observed atconcentrations of 5 or 10 millimoles per liter.

Many of the already known conversion accelerators as described above arenitrogenous compounds. These nitrogenous compounds are refractory toremoval by chemical wastewater treatment methods and must be removed bymicrobiological treatments. However, microbiological treatments havetrouble removing high concentrations of nitrogenous compounds and cannotcompletely remove even low concentrations. Nitrogenous compounds haverecently been one factor contributing to the eutrophication of bodies ofwater and have therefore been targeted for increasingly stringentdischarge regulations. These environmental considerations have createddemand for the development of a nitrogenous compound-free zincphosphate-based conversion treatment bath.

At present, zinc phosphate-based conversion treatments and chromatetreatments are widely used to provide underpaint coatings for thepurpose of improving the post-painting corrosion resistance and paintfilm adherence of various metals. Metal substrates of iron and compositematerials comprising combinations of different materials are primarilysubjected to zinc phosphate-based conversion treatments due to thedifficulties encountered in the chromate treatment of these types ofsubstrates.

The size of the crystals in the coatings afforded by zincphosphate-based conversion treatment generally undergo large variationsas a function of the treatment conditions. Thick coatings of coarsecrystals are satisfactory when the goal is rust prevention or coldworking. However, such coatings do not afford a satisfactory paint filmadherence when they are subsequently painted, and the zincphosphate-based conversion films employed as underpaint coatings must infact be thin films of uniform, fine, and dense film crystals.

Two methods are known for obtaining thin zinc phosphate-type conversionfilms. One method consists of terminating the film deposition reactionsduring the course of these reactions by interrupting contact with theconversion bath. This method results in incomplete deposition of theconversion film and thus in incomplete coverage of the substrate metal.As a result, not only can rusting occur on the substrate metal duringpost-conversion steps such as the water rinse and drying, but thepost-painting corrosion resistance often will also be unsatisfactory.

The other method consists of generating microfine sizes for the filmcrystals. In this method, the film deposition reactions end with thecoating in a thin film form. As a result, the completed conversion filmentirely covers the substrate metal and this method is thus able toprovide both a satisfactory paint film adherence and post-paintingcorrosion resistance.

The above-described zinc phosphate-based conversion treatmenttechnologies are mainly implemented by immersion and spraying. Immersiontechnologies not only do not provide microfine film crystals, butusually require lengthy conversion treatment times when the treatmenttemperature is not at least 55° C. Spray treatment, on the other hand,does provide film crystals that are somewhat finer sized than inimmersion treatment, but which are still not at a level that provides asatisfactory painting performance. And again, treatment temperatures ofat least 55° C. are required in order to carry out treatment in arelatively short time.

A titanium colloid surface-conditioning treatment must usually beapplied to the metal surface immediately prior to conversion treatmentin order to obtain (a) fine-crystal formation in the coating and (b) areduction in the treatment temperature to 50° C. and below. Thissurface-conditioning treatment activates the surface of the metal workwith the result that, regardless of the use of immersion or spraying,the treatment temperature can be lowered, the treatment time can beshortened, and a fine-sized crystalline film can be formed that providesan entirely satisfactory painting performance. However, management ofthe surface conditioner treatment bath is complicated and this treatmentalso requires additional facilities and an expansion of the treatmentspace. These considerations have quite recently strengthened the demandfor a conversion accelerator that can provide a good-quality conversionfilm on metal surfaces even without the execution of asurface-conditioning step.

Also, the titanium colloid dispersed in the surface-conditioningtreatment bath aggregates with elapsed time after bath preparation,leading to a timewise decline in the surface-conditioning activity.Japanese Patent Publication [Kokoku] Number Sho 62-9190 [9,190/1987]teaches management of the Mg/P₂O₇ ratio in the surface-conditioningtreatment bath in order to increase the stability of the titaniumcolloid, while Japanese Patent Application Laid Open [Kokai orUnexamined] Number Sho 63-18084 [18,084/1988] discloses addition to thesurface-conditioning treatment bath of an organic material as astabilizer for the titanium colloid. Each of these methods, however,suffers from inadequate effects, with the result that in practice agedbath must be discharged and freshly prepared bath must be supplied on acontinuous basis in order to cope with the decline in activity. Thispreparation and management of the surface-conditioning treatment bath iscomplex and labor intensive and entails a major economic burden due toits heavy reagent consumption. And of course, since treatment facilitiesare required in order to implement the surface-conditioning treatment,this raises such issues as maintenance of the facilities and anexpansion of the treatment space.

As a consequence of the various issues discussed above, there hasrecently been a strengthening in demand for the development of a surfacetreatment method that can omit the problematic titanium colloidsurface-conditioning treatment while still being able to equip the metalsurface with the uniform, fine, dense, and thin conversion films thatare optimal as underpaint coatings.

A general example of the treatment method used to form a zincphosphate-type conversion film on metals comprises the execution of thefollowing processes in the given sequence: (1) alkaline degreasing, (2)water rinse, (3) conversion, (4) water rinse, and (5) drain and dry.When the film will be used as an underpaint coating, the conversionprocess (3) is preceded by a surface-conditioning step using a titaniumcolloid treatment bath for the purpose of generating uniform, fine, anddense conversion film crystals.

The first drawback to the prior-art surface treatment technologiesdescribed above is that they use a large number of process steps, thusmaking the overall process quite lengthy. As a result, the necessarytreatment facilities are large and take up substantial space. While thesurface treatment methods described above are structured from 5 or 6step processes, the alkaline degreasing step and water rinse step arethemselves frequently implemented as multistage treatments in order toimprove the cleaning efficiency. This raises equipment costs even moreand in addition causes lower productivity, because even longer times arerequired to complete the overall treatment process.

A second drawback to the prior-art technologies as described above isthat they require the management of a large number of parameters. Asexamples, in the alkaline degreasing step the alkalinity (totalalkalinity, free alkalinity) in the degreasing bath must be managed,while in the conversion step the acid concentration in the treatmentbath (total acidity, free acidity) must be managed. This amplificationof the parameters under management increases the operating overhead. Atthe same time, the cost burden is raised by reagent consumption in theseparate process steps. Finally, the storage stability of a titaniumcolloid dispersion is by no means guaranteed, and it requiresappropriate management and periodic disposal and replenishment.

One method that can be considered for solving these two drawbacks is theexecution of the steps from alkaline degreasing to conversion in asingle process step through the use of a surfactant-containing zincphosphate-based conversion bath that combines degreasing and conversion.However, when degreasing and conversion are run at the same time, theconversion reactions initiate sequentially from those regions of themetal work that have been cleaned. This creates a strong tendency forthe quality and appearance of the resulting conversion film to benonuniform.

Another possibility would be to add the surface conditioner to theconversion treatment bath in expectation of producing asurface-conditioning effect on the metal during treatment in theconversion bath. In this case, however, a surface-conditioning effectmust be completely ruled out, because the titanium colloid mainingredient is unstable in the acid region. Thus, not only will thecombined use of surface conditioner and conversion bath not yieldmicrofine-sized film crystals, through a retardation of the filmdeposition rate it will also lead to an additional emphasizing ofinhomogeneities in the appearance of the conversion film.

In sum, then, there is strong demand for a contraction of the treatmentprocess as currently practiced, a reduction in equipment and reagentcosts, and a simplification in treatment bath management. However, thisdemand has in actuality remained unsatisfied to date due to the hightechnical barriers involved in meeting it.

OBJECTS OF THE INVENTION

The present invention provides a zinc phosphate-based conversiontreatment bath and method for application to metals that can deposituniform, fine, and dense zinc phosphate-type conversion films on thesurface of metal substrates and that can induce a microfine-sizing ofthe conversion film crystals.

In addition, the present invention provides a zinc phosphate-basedconversion treatment bath and treatment method that—even without theexecution on the metal surface of surface conditioning with a surfaceconditioner—can deposit thereon a uniform, fine, and dense zincphosphate-type conversion film that contains microfine crystals that arehighly adherent to paint films and that is effective as an underpaintlayer (undercoat) for paint films.

SUMMARY OF THE INVENTION

The aforesaid objects are achieved by a zinc phosphate-based conversiontreatment bath and treatment method according to the present inventionas described below. A zinc phosphate-based conversion treatment bathaccording to the present invention for application to metalscharacteristically contains zinc ions and phosphate ions as its maincomponents and also contains 50 to 1500 parts per million by weight(hereinafter usually abbreviated as “ppm”) of conversion acceleratorconsisting of at least one organoperoxide.

The total content of rid compounds in the zinc phosphate-basedconversion treatment bath according to the present invention ispreferably limited to 0 to 200 ppm, measured as its stoichiometricequivalent as nitrogen. The said organoperoxide is preferably watersoluble and preferably has a peroxy structure or percarboxyl structure.In addition, the subject organoperoxide is preferably selected fromethyl hydroperoxide, isopropyl hydroperoxide, tert-butyl hydroperoxide,tert-hexyl hydroperoxide, diethyl peroxide, di-tert-butyl peroxide,acetylacetone peroxide, cumene hydroperoxide, tert-butylperoxymaleicacid, peracetic acid, monoperphthalic acid, and persuccinic acid.

A zinc phosphate-based conversion treatment bath according to thepresent invention may also contain surfactant.

The zinc phosphate-based conversion treatment method according to thepresent invention for application to metals is characterized by theformation of a zinc phosphate-type conversion film on the surface of ametal by bringing the metal surface into contact with a conversiontreatment bath that contains zinc ions and phosphate ions as its maincomponents and that also contains 50 to 1500 ppm of conversionaccelerator consisting of at least I organoperoxide.

The total content of nitrogenous compounds in the said treatment bathused in the zinc phosphate-based conversion treatment method accordingto the present invention is preferably limited to 0 to 200 ppm as thenitrogen content. The said conversion accelerator is preferably watersoluble and preferably has a peroxy structure or percarboxyl structure.In addition, the subject conversion bath preferably has a pH from 20 to4.0 and preferably is kept at a temperature of 25° C. to 50° C. Thesurface of the metal may also be subjected to a cleaning stepimmediately before the subject conversion treatment.

A conversion treatment bath used in the zinc phosphate-based conversiontreatment method according to the present invention may also containsurfactant in order to simultaneously effect cleaning and conversioncoating of the metal surface. When a surfactant is used, itsconcentration in the conversion treatment bath is preferably from 0.5 to5 g/L.

DETAILS OF THE INVENTION AND ITS PREFERRED EMBODIMENTS

It has been discovered that (1) an organoperoxide conversion acceleratordid not require the co-use of another prior-art conversion acceleratoror a nitric acid compound, which made possible the formulation of anitrogenous compound-free conversion bath; (2) even without theexecution of a surface-conditioning treatment, a uniform, fine, anddense zinc phosphate-type conversion film could be formed on metalsurfaces when organoperoxide was used as the conversion accelerator; and(3) a good performing zinc phosphate-type conversion film can be formedon metals without narrow restrictions originating with the treatmenttemperature or zinc concentration of the treatment bath. The presentinvention was achieved based on these discoveries.

As stated above, the total content of nitrogenous compounds in theconversion bath according to the present invention is limited to 0 to200 ppm, preferably to 0 to 100 ppm, more preferably to 0 to 50 ppm, andeven more preferably to 0 to 20 ppm, in each case measured as itsstoichiometric equivalent as nitrogen.

The most preferred range for the zinc ions content in a conversion bathaccording to the present invention will vary as a function of theparticular application of the conversion film. The preferred zinc ionscontent in the conversion bath is from 0.5 to 15 grams per liter,hereinafter usually abbreviated as “g/L”.

For example, when the conversion bath according to the present inventionis used to provide an underpaint coating on the metal, the preferredconversion film weight is from about 0.5 to 10.0 grams per square meterof surface treated with the bath, hereinafter usually abbreviated as“g/m²”. Due to this, the preferred concentration range for the zinc ionsin the conversion bath for this application will be from 0.5 to 5.0 g/L.When the zinc ions concentration is less than 0.5 g/L, the resultingzinc phosphate-type conversion film will have a reduced coverage ratioand the post-painting paint film adherence and post-painting corrosionresistance usually will be unsatisfactory. At above 5.0 g/L, thepost-painting paint film adherence in particular is reduced due to acoarsening of the film crystals.

When, on the other hand, the conversion bath will be used for coldworking of the metal treated with i, a thick film with a film weight ofabout 5.0 to 15.0 g/m² is preferably laid down in order to provide aconversion film capable of following the plastic deformation of theworkpiece. In this case, the preferred zinc ions concentration range forthe conversion bath will be from 5.0 to 15.0 g/L. At zinc ionsconcentrations below 5.0 g/L, it can be difficult to obtain film weightsas specified above for this application. The coating weight no longerincreases at above 15.0 g/L, which makes such concentrationsuneconomical.

The zinc ions needed in a composition according to the invention can beprovided by dissolving zinc oxide or zinc hydroxide in the acidcomponent in the conversion bath, or by dissolving a water-soluble salt,for example, zinc phosphate or sulfate in the conversion bath.

The phosphate ions concentration in the conversion bath according to thepresent invention is preferably from 5.0 to 30.0 g/L. The formation of anormal conversion film becomes problematic at values below 5.0 g/L. Theeffects of the phosphate ions no longer increase at above 30.0 g/L,which makes such concentrations uneconomical. The phosphate ions can begenerated by the addition of phosphoric acid or its aqueous solution tothe conversion bath or by dissolution in the conversion bath of a saltof phosphoric acid, such as the sodium, potassium, magnesium, or zincsalt.

A zinc phosphate-based conversion treatment bath according to thepresent invention preferably is an acidic aqueous solution with a pHvalue from 2.0 to 4.0 and more preferably about 2.5 to 3.5. In this pHregion, orthophosphoric acid (H₃PO₄) has an equilibrium relationshipwith dihydrogen phosphate ions (H₂PO₄ ⁻), hydrogen phosphate ions (HPO₄⁻²), and phosphate ions (PO₄ ³⁻), and the stoichiometric equivalent asphosphate ions of all of these species, along with any of the condensedphosphoric acids and their salts in which phosphorus has its +5 valencestate, are considered to be part of the “phosphate ions” content as usedherein, irrespective of whatever degree of ionization may actually existin the composition.

A conversion bath according to the present invention contains conversionaccelerator consisting of at least one selection from theorganoperoxides. This organoperoxide is preferably water soluble and ispreferably selected from compounds having a peroxy structure orpercarboxyl structure. The organoperoxide used by the present inventionencompasses aromatic peroxides, cyclic aliphatic peroxides, andaliphatic peroxides, and aliphatic peroxides having 1 to 7 carbon atomsare preferred. Organoperoxides bearing long-chain alkyl and aromaticperoxides can be inadequately soluble in water and thus can have anunsatisfactory conversion accelerating activity.

Organoperoxides effective as a conversion accelerator are preferablyselected from those with a simple peroxy structure, such as ethylhydroperoxide, isopropyl hydroperoxide, tert-butyl hydroperoxide,tert-hexyl hydroperoxide, diethyl peroxide, di-tert-butyl peroxide,acetylacetone peroxide, cumene hydroperoxide, and tert-butylperoxymaleicacid, and those with a percarboxyl structure, such as peracetic acid,monoperphthalic acid, and persuccinic acid.

When the organoperoxide has a low solubility in the conversion bath, thepoorly soluble compound can be solubilized by the addition to thetreatment bath of a small amount of water-soluble organic solvent, forexample, tert-butyl alcohol or isopropyl alcohol.

A working conversion treatment bath according to the present inventionas described above preferably contains the conversion accelerator at aconcentration of 50 to 1,500 ppm and preferably 80 to 1,200 ppm. Theconversion accelerating activity will usually be unsatisfactory when theconversion accelerator concentration is less than 50 ppm. The conversionaccelerating activity no longer increases at conversion acceleratorconcentrations above 1,500 ppm, which makes such concentrationsuneconomical.

Since the conversion treatment bath according to the present inventionalso has the ability to microfine-size the deposited zinc phosphate-typecrystals, it can produce a uniform, fine, and dense zinc phosphate-typeconversion film even in the absence of any preceding surfaceconditioning treatment for the purpose of microfine-sizing the filmcrystals. Moreover, since the conversion treatment bath according to thepresent invention need not contain nitric acid, nitrous acid, an organicnitro compound, etc., it can be formulated completely free ofnitrogenous compounds. In this case, effluent treatment will not requirea process for treating nitrogenous compounds. Although the addition ofnitrogenous compounds to the conversion bath according to the presentinvention is not precluded, the nitrogen concentration is preferablylimited as discussed above to 0 to 200 ppm.

In addition to zinc ions, a zinc phosphate-based conversion bathaccording to the present invention may also contain supplementary metalions. These supplementary metal ions can function as an etchant in orderto induce a uniform etch of the surface of the metal substrate, or, inthe case of application as an underpaint coating, they can function toimprove the painting performance.

Such non-zinc supplementary metal ions can be nickel ions, manganeseions, cobalt ions, iron ions, magnesium ions, calcium ions, and soforth. These supplementary metal ions can be provided in a compositionaccording to the invention by dissolving their oxides, hydroxides,carbonates, sulfates, phosphates, etc., in the treatment bath.

Supplementary metal ions can be added to the conversion bath accordingto the present invention at 100 to 3,000 ppm and preferably at 200 to2,000 ppm.

When ferriferous material is treated with a conversion bath according tothe present invention, trivalent iron ions will dissolve from the metalinto the treatment bath and will accumulate at levels of 10 to 50 ppm.The accumulation of this amount of trivalent iron ions does not have anegative influence on the effects of the treatment bath and methodaccording to the present invention. Accordingly, trivalent iron ions maybe added to or may be present in the treatment bath within this rangeprior to conversion treatment.

Depending on the particular requirements, a conversion bath according tothe present invention may contain fluoride ions or fluorine-containinganions, for example, complex fluoride ions such as fluosilicate ions orfluozirconate ions. Fluorine-containing anions can be provided in acomposition according to this invention by dissolving afluorine-containing compound in the conversion bath, for example,hydrofluoric acid, fluosilicic acid, fluozirconic acid, fluotitanicacid, and their metal salts (sodium salts, potassium salts, magnesiumsalts).

A method according to the present invention includes a process in whichthe surface of the metal is brought into contact with the zincphosphate-based conversion treatment bath. When the metal already has aclean surface, the zinc phosphate-based conversion treatment can bedirectly executed on the clean metal by the method according to thepresent invention. However, when the surface of the metal work iscontaminated with microscopic metal particles, dust, or grease, thecontaminants should preferably be removed from the metal surface priorto the conversion treatment, by executing a cleaning treatment on themetal surface, preferably a cleaning treatment using a waterbornealkaline degreasing bath, waterborne cleaning emulsion, or cleaningsolvent. When a waterborne cleaning bath is used, any of it remaining onthe surface is preferably removed by rinsing the metal surface withwater.

In general, prior to conversion treatment the surface of the metal isdegreased with an alkaline degreaser and then rinsed with water. Inaddition, after the conversion treatment the conversion film is rinsedwith water and then dried. Both the degreasing and rinse processes maybe implemented as multistage processes. When the conversion film is tobe used as an underpaint coating, the final rinse preferably usesdeionized water.

In addition, when the conversion film is placed on the metal surface tofunction as an undercoating for paint films, a surface-conditioningtreatment using a titanium compound colloid-containing surfaceconditioner is preferably executed on the metal surface immediatelyprior to the conversion treatment. However, this surface-conditioningtreatment can be omitted in the method according to the presentinvention.

The conversion treated surface of the metal is rinsed with water, driedas necessary, and then painted.

When the conversion treatment bath according to the present inventionwill be used to lay down a conversion film in order to support coldworking of the metal, the degreasing and water rinse steps arepreferably followed by an acid rinse of the metal in order to removescale from the metal surface.

When the conversion film is to be used to support cold working, the filmsurface is preferably lubricated with a lubricant, for example, a soap,in order to improve the lubricating properties of the conversion film.

Contact between the metal being treated and the conversion treatmentcomposition in a method according to the present invention is generallyeffected by, for example, immersion, spraying, or a combination thereof.When the conversion treatment is being run in order to provide anundercoat for paint films, the treatment is preferably run for 0.5 to 5minutes at a temperature from ambient temperature to 60° C. When theconversion treatment is being run on metal that will be cold worked, thetreatment temperature is preferably from 50° C. to 90° C. and thetreatment time is preferably from 1 to 15 minutes. The above-describedtreatment conditions will yield the desired conversion films.

Because the organoperoxide (conversion accelerator) in the conversionbath according to the present invention functions as an oxidizer, itsreaction and/or decomposition products will accumulate in the treatmentbath. For example, alcohol is produced by the reaction and/ordecomposition of hydroperoxide, while alcohol and carboxylic acid areproduced by the reaction and/or decomposition of peroxyester. Carboxylicacid is also produced by the reaction and/or decomposition ofpercarboxylic acid. The accumulation of these reaction and/ordecomposition products does not exercise a negative influence on thetreatment bath and method according to the present invention. As aconsequence, prior to conversion treatment the reaction and/ordecomposition products of the organoperoxide may be present in thetreatment bath according to the present invention, or may even be addedto the bath, in either case without normally causing any problems.

The type, form, and dimensions of metal substrates that may be subjectedto the method according to the present invention are entirelyunrestricted.

In specific terms, the method according to the present invention can beapplied to various ferriferous materials, for example, steel sheet andsteel sheet plated with zinciferous metal, and to various aluminiferousmetals, for example, aluminum and aluminum alloys such asaluminum-magnesium alloys and aluminum-silicon alloys.

A zinc phosphate-based conversion treatment bath according to thepresent invention may as necessary also contain surfactant for cleaningthe surface of the metal.

The metal surface can be cleaned when surfactant is present in theconversion bath and, concurrently with this, can be covered with a zincphosphate conversion film. The surface of the metal may be soiled inthis case, and there are absolutely no restrictions on thesecontaminants as long as they can be removed by the surfactant-containingconversion bath. These contaminants include oils and greases, forexample, grease, antirust oils, and press oils (these may becontaminated with dust); microfine metal particles; and other material.The amount of contaminant is also not narrowly restricted.

Surfactant usable in the present invention comprises at least oneselection from the nonionic, cationic, anionic, and amphotericsurfactants. However, cationic surfactant/anionic surfactantcombinations should be avoided due to the corresponding problems withtreatment bath stability.

Nonionic surfactants usable in the method according to the presentinvention are exemplified by polyethylene glycol-type nonionicsurfactants such as polyoxyethylene alkylphenyl ethers, polyoxyethylenealkyl ethers, polyoxyethylene fatty acid esters, polyoxyethylenesorbitan fatty acid esters, polyoxyethylene-polyoxypropylene blockpolymers, and so forth; polyvalent alcohol-type nonionic surfactantssuch as sorbitan fatty acid esters and so forth; and amide-type nonionicsurfactants such as fatty acid alkylolamides and so forth.

Cationic surfactants usable in the method according to the presentinvention are exemplified by amine salt-type cationic surfactants suchas the salts of higher alkylamines, polyoxyethylene higher alkylaminesalts, and so forth and by quaternary ammonium salt-type cationicsurfactants such as alkyltrimethylammonium salts and so forth.

Amphoteric surfactants usable in the method according to the presentinvention are exemplified by amino acid-type amphoteric surfactants suchas methyl alkylaminopropionate and so forth and by betaine-typeamphoteric surfactants such as alkyldimethylbetaines and so forth.

Anionic surfactants, however, generally have low solubilities in theacid region and for this reason their use in the present invention isfrequently problematic. However, types in which ethylene oxide has beenadded, as in the higher alkyl ether sulfate ester salts, can be usedsince they retain good solubilities even in the acid region.

Concentrations of about 0.5 to 5 g/L are suitable for these surfactantsin a zinc phosphate-based conversion treatment bath in the methodaccording to the present invention. The type and concentration of thesurfactant should be selected as appropriate as a function of the typeand concentration (add-on) of the oil, grease, or other soil to becleaned off.

The surface is cleaned at the same time as its conversion treatment whensurfactant is present in the bath. When this method is run continuously,the cleaned off soil will usually therefore accumulate in the treatmentbath. Since this accumulated soil is not inevitably benign ornondetrimental for the conversion treatment, its total accumulation ispreferably limited to no more than 10 g/L. This restriction on the totalaccumulation will, however, vary as a function of the type of soil andthe type and content of the surfactant.

After a conversion treatment with the surfactant-containing conversionbath, the resulting conversion film is rinsed with water and theresidual water is eliminated from the surface of the conversion film.The water rinse may be implemented as a single-step or multistepprocess, but the final water rinse preferably uses deionized water.

The aforesaid water elimination process (drying process) is not anabsolute requirement when the conversion film-carrying surface of themetal is to be coated with paint, for example, by electrodeposition.There are absolutely no restrictions on the drying temperature or time,e.g., drying can be carried out at room temperature or with heating.

This treatment of the metal surface with surfactant-containingconversion bath according to the present invention provides a thoroughelimination of the oil, grease, dust, and/or metal particles and, at thesame time as this cleaning, accelerates the conversion film-formingreactions through the presence of the conversion accelerator(organoperoxide).

Thus, the surface of the metal will be cleaned and, concurrently withthis cleaning, a uniform, fine, and dense zinc phosphate-type conversionfilm having microfine film crystals will be formed on the cleaned metalsurface.

The invention will be explained in greater detail below using workingexamples, which, however, are provided simply for purposes ofexplanation and should not be construed as limiting the scope of theinvention.

EXAMPLES 1 TO 8 AND COMPARATIVE EXAMPLES 1 TO 4

The following metals were used in these working and comparativeexamples.

(1) Cold-rolled steel sheet (SPCC-SD, abbreviated below as SPC) with asheet thickness of 0.8 mm.

(2) Galvanized steel sheet (abbreviated in the table as “plated”)afforded by electrogalvanizing, to an add-on mass of 20 g/m², the typeof cold-rolled steel sheet described in (1).

These metals were each cut into 70×150 mm coupons.

In these examples and comparative examples, conversion films were formedon the above-described metals using the following process sequence,unless otherwise stated. These films were intended for application asunderpaint coatings (undercoats):

(1) Degreasing (FINECLEANER® L4460 alkaline degreaser, from NihonParkerizing Company, Limited, 20 g/L of agent A, 12 g/L of agent B) at43° C. for 120 seconds by immersion;

(2) Water rinse with tap water at ambient temperature for 30 seconds,spray;

(3) Surface conditioning (colloidal titanium surface conditioner,trademark: PREPALENE® ZN from Nihon Parkerizing Company, Limited, 1 g/Laqueous solution), at ambient temperature for 30 seconds, spray;

(4) Zinc phosphate-based conversion treatment, with compositionsdescribed in the individual working and comparative examples, at 43° C.for 120 seconds immersion;

(5) water rinse, with tap water at ambient temperature for 30 seconds,spray;

(6) deionized water rinse (deionized water, conductivity=0.2microSiemens/cm) at ambient temperature for 20 seconds, spray;

(7) drain and dry in a hot air current at 110° C. for 180 seconds.

However, in Examples 5 and 7 and Comparative Example 3, thesurface-conditioning treatment in (3) was not run and the degreased andwater rinsed metal surface was submitted to the zinc phosphate-basedconversion treatment as in step (4) directly after degreasing (1) andthe water rinse in step (2).

The free acidity in the zinc phosphate-based conversion baths inExamples 1 to 8 and Comparative Examples 1 to 4 was adjusted to thespecified values, vide infra, using sodium hydroxide. The free aciditywas measured by titrating 10 milliliters, hereinafter usuallyabbreviated as “mL”, of the particular treatment bath to neutralityusing 0.1 N aqueous sodium hydroxide and bromophenol blue as theindicator. The number of milliliters (mL) of the aqueous sodiumhydroxide solution required for the color change from yellow to blue wasdetermined and is reported as “points” of free acidity The fluoride ionsconcentration in the conversion baths was measured using a fluorine ionsensitive electrode.

The coating weight was measured as follows. The weight (“W1”) in gramsof the treated coupon after conversion treatment was first measured, andthe treated coupon was then subjected to a film stripping treatmentusing the stripping solution and stripping conditions reported below.The weight of the stripped coupon was measured to give “W2” in grams,and the coating mass in g/m² was calculated from the formula(W1−W2)/(0.021).

Treatment for cold-rolled steel coupons

stripping solution: 5% by weight of aqueous chromic acid solution

stripping conditions: 75° C., 15 minutes, immersion.

Treatment for galvanized steel coupons

stripping solution: 2% by weight of ammonium dichromate +49% by weightof 28% aqueous ammonia+49% by weight of pure water

stripping conditions: ambient temperature, 15 minutes, immersion.

The appearance of the coatings was inspected visually, and themorphology and size of the grains in the conversion coating wereevaluated by inspection with a scanning electron microscope (“SEM”).

Conversion treatment bath (1) with the following composition wasprepared in Example 1.

Composition of conversion treatment bath (1)

phosphate ions   15 g/L (from addition of 75% phosphoric acid) zinc ions 1.3 g/L (from addition of zinc oxide) nickel ions  1.0 g/L (fromaddition of nickel carbonate) manganese ions  0.5 g/L (from addition ofmanganese carbonate) fluoride ions  100 ppm (from addition of 55%hydrofluoric acid)

450 ppm of tert-butyl hydroperoxide was added as the organoperoxide tothe conversion bath with the above composition, and the free acidity ofthe conversion bath was then adjusted to 0.9 point. A cold-rolled steeltest coupon was subjected first to the colloidal titaniumsurface-conditioning treatment (3) and then to conversion treatment at atemperature of 43° C. for 120 seconds, using the above-describedconversion bath (1). The resulting conversion coating weight was 1.2g/m². The coating crystals were plates with an average grain size of 6micrometers. The conversion coating was grayish black and was uniform,fine, and dense. Other test results are reported in Table 1.

TABLE 1 Surface Identifi- Conc. in Treatment Comp. of: Condi- Acceler-cation Substrate PO₄ ⁻³, g/L Zn⁺², g/L N, ppm tioner? ator Used Ex 1 SPC15 1.3 0 yes a Ex 2 plated 15 1.3 0 yes a Ex 3 SPC 15 1.3 0 yes a Ex 4SPC 15 1.3 500  yes a Ex 5 SPC 15 1.3 0 no b Ex 6 SPC 15 1.3 0 yes c Ex7 SPC 15 1.3 0 no a Ex 8 SPC 15 1.3 1400   yes a CE 1 SPC 15 1.3 0 yes aCE 2 plated 15 1.3 0 yes a CE 3 SPC 15 1.3 1400   no d CE 4 SPC 15 1.3 0yes e Conc. Points Identi- of of Coating Coating Coating fica- Acc.,Free Mass, Crystal Grain tion ppm Acid g/m² Coating Appearance ShapeSize, μm Ex 1 450 0.9 1.2 Grayish black plates 6 Ex 2 450 0.9 2.8grayish white plates 4 Ex 3  80 0.6 0.9 grayish black plates 8 Ex 41200  0.9 1.1 grayish black plates 7 Ex 5 400 0.9 1.0 grayish blackplates 6 Ex 6 100 0.6 1.3 grayish black plates 10  Ex 7 500 0.6 1.1grayish black plates 10  Ex 8 450 0.9 1.1 grayish black plates 5 CE 1  50.9 0.5 yellow rust appeared columnar 13  CE 2  5 0.9 0.9 sparse coatingcolumnar 15  CE 3 150 0.9 0.1 yellow rust appeared granular 80  CE 41500  0.9 0.9 sparse coating columnar 15  Abbreviations in, and OtherNotes for, Table 1 “Ex” means “Example”; “CE” means “ComparativeExample”; “Conc.” means “Concentration”; “Comp.” means “Composition”;“Acc.” means “Accelerator”; “μm” means “micrometers”. In the columnheaded “Accelerator Used”: “a” means tert-butyl hydroperoxide; “b” meanstert-hexyl hydroperoxide; “c” means peracetic acid; “d” means nitriteions; “e” means chlorate ions.

In Example 2, a galvanized steel test coupon was subjected first to thesame degreasing (1), water rinse (2), and surface-conditioning treatment(3) as in Example I and then to conversion treatment as in Example 1using conversion treatment bath (1). The resulting conversion coatingweight was 2.8 g/m². The crystals were plates with an average grain sizeof 4 micrometers. The conversion coating was grayish white and wasuniform, fine, and dense.

In Example 3, a cold-rolled steel test coupon was subjected first to thesame surface-conditioning treatment as in Example 1 and then toconversion treatment using the same conversion treatment bath as inExample 1, except that the organoperoxide consisted of 80 ppm tert-butylhydroperoxide and the free acidity was adjusted to 0.6 point. Theresulting conversion coating weight was 0.9 g/m². The crystals wereplates with an average grain size of 8 micrometers. The conversioncoating was grayish black and was uniform, fine, and dense.

In Example 4, a cold-rolled steel test coupon was subjected first to thesame surface-conditioning treatment as in Example 1 and then toconversion treatment using the same conversion treatment bath as inExample 1, except that 1200 ppm of tertbutyl hydroperoxide was theorganoperoxide and sufficient 65.5% nitric acid was added to give anitrogen component content of 500 ppm. The free acidity of theconversion bath was adjusted to 0.9 point. The resulting conversioncoating weight was 1.1 g/m². The coating crystals were plates with anaverage grain size of 7 micrometers. The conversion coating was grayishblack and was uniform, fine, and dense.

In Example 5, a cold-rolled steel test coupon, without anysurface-conditioning treatment, was subjected to conversion treatment asin Example 1, except that 400 ppm of tert-hexyl hydroperoxide was theorganoperoxide. The free acidity was adjusted to 0.9 point. Theresulting conversion coating weight was 1.0 g/m². The coating crystalswere plates with an average grain size of 6 micrometers. The conversioncoating was grayish black and was uniform, fine, and dense.

In Example 6, a cold-rolled steel test coupon was subjected first to thesame surface-conditioning treatment as in Example 1 and then toconversion treatment using the same conversion treatment bath as inExample 1, except that 100 ppm of peracetic acid was the organoperoxide,and the free acidity was adjusted to 0.6 point. The resulting conversioncoating weight was 1.3 g/m². The coating crystals were plates with anaverage grain size of 10 micrometers. The conversion coating was grayishblack and was uniform, fine, and dense.

In Example 7, a cold-rolled steel test coupon, without any surfaceconditioning treatment, was subjected to conversion treatment using thesame conversion bath as in Example 1, except that 500 ppm of tert-butylhydroperoxide was added as the organoperoxide, and the free acidity wasadjusted to 0.6 point. The resulting conversion coating weight was 1.1g/m². The coating crystals were plates with an average grain size of 10micrometers. The conversion coating was grayish black and was uniform,fine, and dense.

Conversion treatment bath (2) with the following composition wasprepared for Example 8.

Composition of conversion treatment bath (2)

phosphate ions   15 g/L (from addition of 75% phosphoric acid) zinc ions 1.3 g/L (from addition of zinc oxide) nickel ions  1.0 g/L (fromaddition of nickel carbonate) manganese ions  0.5 g/L (from addition ofmanganese carbonate) fluoride ions  100 ppm (from addition of 55%hydrofluoric acid) nitrate ions  7.2 g/L (from addition of sodiumnitrate and nickel     nitrate)     (nitrogen concentration = 1.4 g/L)

450 ppm of tert-butyl hydroperoxide was added as the organoperoxide tothe conversion bath with the above composition, and the free acidity ofthe conversion bath was then adjusted to 0.9 point. A cold-rolled steeltest coupon was subjected first to the colloidal titaniumsurface-conditioning treatment and then to conversion treatment(conversion temperature=43° C., treatment time=120 seconds) using theabove-described conversion bath. The resulting conversion coating weightwas 1.1 g/m². The coating crystals were plates with an average grainsize of 5 micrometers. The conversion coating was grayish black and wasuniform, fine, and dense.

In Comparative Example 1, a cold-rolled steel test coupon was subjectedto the same surface-conditioning treatment as in Example I and was thensubmitted to the same conversion treatment as in Example 1, except thatthe organoperoxide consisted of 5 ppm of tert-butyl hydroperoxide. Theconversion coating weight was 0.5 g/m², and the development of yellowrust was observed.

In Comparative Example 2, a galvanized steel test coupon was subjectedto conversion treatment as in Example 1, except that the organoperoxideconsisted of 5 ppm of tert-hydroperoxide. The conversion coating weightwas 0.9 g/m², the average grain size was 15 micrometers, and the coatingwas sparse.

In Comparative Example 3, a cold-rolled steel test coupon, without anysurface-conditioning treatment, was subjected to conversion treatment asin Example 8, except that 150 ppm of nitrite ions were added to theconversion bath in place of the organoperoxide. The conversion coatingweight was 0.1 g/m², which indicated that almost no conversion coatingdeposition had occurred. Yellow rust had developed over the entiresurface.

In Comparative Example 4, a cold-rolled steel test coupon was subjectedto conversion treatment as in Example 1. In this case, however, sodiumchlorate was added to the conversion bath in place of the organoperoxideto give a chlorate ions concentration of 1.5 g/L. The conversion coatingweight was 0.9 g/m². The coating crystals were columnar and the averagegrain size was 15 micrometers. The conversion coating was sparselydeposited, and yellow rust was observed.

The test results are reported in Table 1. The organoperoxideconcentrations used in Examples 1 to 8 were within the range from 50 to1,500 ppm. It was thereby confirmed that this concentration rangeproduced a good-quality conversion coating on cold-rolled steel sheet aswell as galvanized steel sheet. A uniform, dense, and fine coating wasobtained even when the surface-conditioning treatment was not used.

In contrast, Comparative Examples 1 and 2 used organoperoxideconcentrations below 50 ppm, and it was confirmed that in these casesthe oxidation activity by the conversion accelerator was inadequate,resulting in the deposition of only scattered coating crystals. Theuniformity of the coating on the substrate metal was thereforediminished.

Comparative Examples 3 and 4 used non-organoperoxide conversionaccelerators. In Comparative Example 3, nitrite ions were used as theconversion accelerator and no surface-conditioning treatment was carriedout. It was confirmed that in this case conversion coating depositionwas entirely absent. Chlorate ions were used by themselves as theconversion accelerator in Comparative Example 4. It was confirmed thatin this case the conversion reaction rate was substantially slowed.

EXAMPLES 9 TO 15

The following metals were used in these examples:

(1) Cold-rolled steel sheet (SPCC-SD, sheet thickness: 0.8 mm,abbreviated below as “SPC”)

(2) Zinc-electroplated steel sheet (sheet thickness: 0.8 mm, platingweight: both surfaces 30 g/m², abbreviated below as “EG”)

(3) Galvannealed hot-dip zinc-plated steel sheet (sheet thickness: 0.8mm, plating weight: both surfaces 45 g/m², abbreviated below as “GA”)

(4) Aluminum-magnesium alloy sheet (Japanese Industrial Standard-A5052,sheet thickness: 1.0 mm, abbreviated below as “AL”).

In each case the metals were cut to 70×150 mm to prepare the specimensthat were then subjected to the treatments in the working andcomparative examples. Each test material was coated with 2 g/m² of acommercial cleaning/rust-preventing oil.

The same treatments as in Example 1 were executed on the metal specimensin each of Examples 9 to 15 with the following modifications: thesurface-conditioning treatment was omitted and conversion baths (3),(4), and (5) with the compositions given below were used in place ofconversion bath (1).

Composition of Conversion Treatment Bath (3)

phosphate ions  15 g/L (from addition of 75% phosphoric acid) zinc ions1.3 g/L (from addition of zinc oxide) nickel ions 0.5 g/L (from additionof nickel carbonate) fluorine component 1.0 g/L (from addition of sodiumfluosilicate) 2-butanol  30 g/L conversion accelerator see below freeacidity 0.6 point

Composition of Conversion Treatment Bath (4)

phosphate ions  13 g/L (from addition of 75% phosphoric acid) zinc ions1.1 g/L (from addition of zinc oxide) cobalt ions 0.4 g/L (from additionof basic cobalt carbonate) fluorine component 0.4 g/L (from addition ofsodium fluoride) conversion accelerator see below free acidity 0.4 point

Composition of Conversion Treatment Bath (5)

phosphate ions  17 g/L (from addition of 75% phosphoric acid) zinc ions1.5 g/L (from addition of zinc oxide) conversion accelerator see belowfree acidity 0.7 point

Each of the conversion baths (3) to (5) was adjusted to the specifiedfree acidity using sodium hydroxide. Otherwise, the free acidity(points), conversion coating weight, and status and size of the coatingcrystals were measured as described above.

Standards for Reporting the Evaluation of the Grain Size of the CoatingCrystals

(1) for cold-rolled steel sheet:

+ less than 35 micrometers

x greater than or equal to 35 micrometers

(2) for zinc-electroplated steel sheet

+ less than 25 micrometers

x greater than or equal to 25 micrometers

(3) for galvannealed hot-dip zinc-plated steel sheet:

+ less than 30 micrometers

x greater than or equal to 30 micrometers

(4) for aluminum-magnesium alloy sheet:

+ less than 30 micrometers

x greater than or equal to 30 micrometers

Standard for Reporting Evaluation of Substrate Metal Coverage

Considered over the entire material:

+ absolutely no exposure of substrate metal

x exposure of substrate metal was observed

Conversion-treated test panels were electrodeposition painted using acationic electrodeposition paint (Elecron™ 2000 from Kansai PaintKabushiki Kaisha) to give a paint film with a film thickness of 20micrometers. These painted specimens were then subjected to thefollowing painting performance tests in order to evaluate the paintingperformance:

(1) Test of the Post-painting Corrosion Resistance

A cut was introduced into the paint film on the painted sample. Thepainted sample was thereafter immersed for 240 hours in 5% aqueoussodium chloride solution heated to 50° C. and then removed, rinsed withwater, and dried. The neighborhood of the cut was peeled usingcellophane tape, and the maximum width of paint film peeling on one sidewas measured after the tape peel and reported on the following scale:

+ maximum one-side width of peel is less than 7 mm # maximum one-sidewidth of peel is at least 7 mm but less than 10 mm × maximum one-sidewidth of peel is at least 10 mm

(2) Test of the Water-resistant Secondary Adherence

The painted sample was immersed for 240 hours in pure water heated to40° C. and then removed and dried. A cross was thereafter scribed in thepaint film; the center of the cut was extruded 3 mm using an Erichsentester; and, after a cellophane tape peel, the paint film peel ratio(peeled area/extruded area) was measured. The following scale was usedfor reporting:

+ paint film peel ratio is less than 10% # paint film peel ratio is atleast 10% but less than 20% × paint film peel ratio is at least 20%

In Example 9, 200 ppm of tertbutyl hydroperoxide was added as conversionaccelerator to conversion treatment bath (3), which was then used toconversion treat the cold-milled steel sheet by immersion at a treatmenttemperature of 45° C. The treatment conditions and test results arereported in Tables 2 and 3, respectively.

In Example 10, 80 ppm of di-tert-butyl peroxide was added as conversionaccelerator to conversion treatment bath (3), which was then used toconversion treat the zinc-electroplated steel sheet by immersion at atreatment temperature of 45° C. The treatment conditions and testresults are reported in Tables 2 and 3, respectively.

TABLE 2 Treat- Other Free ment Con- PO₄ ⁻³, Zn⁺², Metal Perox- AcidTemp., tact Ex # Sub. g/L g/L Ion, g/L ide, g/L F, g/L Points ° C.Method 9 SPC 15 1.3 Ni:0.5 a:200 1.0 0.6 45 imm. 10 EG 15 1.3 Ni:0.5f:80 1.0 0.6 45 imm. 11 SPC 13 1.1 Co:0.4 a:500 0.4 0.4 40 spray 12 EG13 1.1 Co:0.4 g:1100 0.4 0.4 40 imm. 13 SPC 15 1.3 Ni:0.5 f:500 1.0 0.643 imm. 14 GA 17 1.5 — a:500 — 0.7 33 spray 15 AL 15 1.3 Ni:0.5 f:1501.0 0.6 43 spray Additional Abbreviations in and Other Notes for Table 2“#” means “Number”; “Temp.” means “Temperature”; “imm.” means“immersion. In the column headed “Peroxide, g/L”: “a” means “tert-butylhydroperoxide”; “f” means “di-tert-butyl peroxide”; “g” means“acetylacetone peroxide”.

TABLE 3 Coating Post-Painting Water-Resistant Example Coating Grain SizeCoverage Corrosion Secondary Adherence Number Mass, g/m² Rating RatingRating Rating 9 0.9 + + + + 10 3.5 + + + + 11 1.2 + + + + 12 3.2 + + + +13 1.3 + + + + 14 4.3 + + # + 15 2.5 + + + +

In Example 11, 500 ppm of tert-butyl hydroperoxide was added asconversion accelerator to conversion treatment bath (4), which was thenused to conversion treat the cold-rolled steel sheet by spraying at atreatment temperature of 40° C. The treatment conditions and testresults are reported in Tables 2 and 3, respectively.

In Example 12, 1100 ppm of acetylacetone peroxide was added as coneversion accelerator to conversion treatment bath (4), which was thenused to conversion treat the zinc-electroplated steel sheet by immersionat a treatment temperature of 40° C. The treatment conditions and testresults are reported in Tables 2 and 3, respectively.

In Example 13, 500 ppm of di-tert-butyl peroxide was added as conversionaccelerator to conversion treatment bath (3), which was then used toconversion treat the cold-rolled steel sheet by immersion at a treatmenttemperature of 43° C. The treatment conditions and test results arereported in Tables 2 and 3, respectively.

In Example 14, 500 ppm of tertbutyl hydroperoxide was added asconversion accelerator to conversion treatment bath (5), which was thenused to conversion treat the galvannealed hot-dip zinc-plated steelsheet by spraying at a treatment temperature of 33° C. The treatmentconditions and test results are reported in Tables 2 and 3,respectively.

In Example 15, 150 ppm of di-tert-butyl peroxide was added as conversionaccelerator to conversion treatment bath (3), which was then used toconversion treat the aluminum-magnesium alloy sheet by spraying at atreatment temperature of 43° C. The treatment conditions and testresults are reported in Tables 2 and 3, respectively.

COMPARATIVE EXAMPLES 5 TO 9

In each of Comparative Examples 5 to 9, the same treatments and testswere run as in Example 9, with the exception of the modifications givenbelow.

In Comparative Example 5, 200 ppm nitrite ions was added as conversionaccelerator to conversion treatment bath (3), which was then used toconversion treat the cold-rolled steel sheet by immersion in thetreatment bath at a treatment temperature of 43° C. The treatmentconditions and test results are reported in Tables 4 and 5,respectively.

TABLE 4 Other Treat- Metal Accel- Free ment Con- PO₄ ⁻³, Zn⁺², Ions,erator, Acid Temp., tact CE # Sub. g/L g/L g/L g/L F, g/L Points ° C.Method 5 SPC 15 1.3 Ni:0.5 d:200 1.0 0.6 43 imm. 6 SPC 15 1.3 Ni:0.5 —1.0 0.6 43 imm. 7 EG 13 1.1 Co:0.4 e:2000 0.4 0.4 40 imm. 8 GA 17 1.5 —— — 0.7 33 spray 9 AL 15 1.3 Ni:0.5 — 1.0 0.6 43 spray Additional Notesfor Table 4 In the column headed “Accelerator, g/L”: d: nitrite ions e:chlorate ions

TABLE 5 Comparative Coating Post-Painting Water-Resistant ExampleCoating Grain Size Coverage Corrosion Secondary Adherence Number Mass,g/m² Rating Rating Rating Rating 5 4.0 x x # x 6 0.5 x x x # 7 5.2 x + #x 8 7.3 x + x x 9 1.3 x x x #

In Comparative Example 6, conversion treatment bath (3)—without theaddition of conversion accelerator—was heated to 43° C., and thecold-rolled steel sheet was conversion treated by immersion in thistreatment bath. The treatment conditions and test results are reportedin Tables 4 and 5, respectively.

In Comparative Example 7, 2000 ppm of chlorate ions was added asconversion accelerator to conversion treatment bath (4), which was thenused to conversion treat the zinc-electroplated steel sheet by immersionat a treatment temperature of 40° C. The treatment conditions and testresults are reported in Tables 4 and 5, respectively.

In Comparative Example 8, conversion treatment bath (5)—without theaddition of conversion accelerator—was heated to 33° C., and thegalvannealed hot-dip zinc-plated steel sheet was conversion treated byspraying with this bath. The treatment conditions and test results arereported in Tables 4 and 5, respectively.

In Comparative Example 9, conversion treatment bath (3)—without theaddition of conversion accelerator—was heated to 43° C., and thealuminum-magnesium alloy sheet was conversion treated by spraying withthis bath. The treatment conditions and test results are reported inTables 4 and 5, respectively.

Examples 9 to 15, which employed a surface treatment method according tothe present invention, consisted of treatment using a conversiontreatment bath that contained organoperoxide as the conversionaccelerator. As Tables 2 to 5 clearly show, in each case this resultedin the deposition of a thin, uniform, fine, and dense zincphosphate-type conversion coating on the surface of the metal work andin an excellent painting performance (post-painting corrosion resistanceand water-resistant secondary adherence). In Comparative Examples 6, 8,and 9, treatment was carried out using a conversion treatment bath thatwas entirely free of conversion accelerator. In contrast to theexamples, the oxidizing activity in these comparative examples wasinadequate, and only sparse coating crystals were deposited and thesubstrate metal was not uniformly covered. Comparative Examples 5 and 7employed, respectively, nitrite ions, which are the conversionaccelerator most typically used in the prior art, and chlorate ions.Fine, dense films were not deposited in these comparative examples and asatisfactory painting performance was therefore not obtained.

EXAMPLES 16 TO 22 AND COMPARATIVE EXAMPLES 10 TO 14

Examples 16 to 22 and Comparative Examples 10 to 14 employed the samecold-rolled steel sheet (SPC sheet) as in Example 9, the samezinc-electroplated steel sheet and galvannealed hot-dip zinc-platedsteel sheet (sheet thickness: 2.8 mm, plating weight: both surfaces 45g/m²) as in Example 10, and the same aluminum-magnesium alloy sheet asin Example 15. The metal sheets were coated with 2 g/m² of a commercialcleaning/rust-preventing oil (NOX-RUST™ 550 from Parker Kosan KabushikiKaisha).

The treatment processes common to Examples 16 to 22 and ComparativeExamples 10 to 14 are given below.

(1) cleaning/conversion treatment

The specific conditions are given below in the respective working andcomparative examples.

(2) tap-water rinse

ambient temperature, 30 seconds, spray

(3) deionized water rinse

deionized water with a conductivity of 0.2 microSiemens/cm ambienttemperature, 20 seconds, spray

(4) drain/dry

hot air current at 110° C. for 180 seconds

Each of the cleaning/conversion treatment baths used in the working andcomparative examples was adjusted to the specified free acidity, videinfra, using sodium hydroxide unless specified otherwise. The freeacidity (in points) of the treatment baths was measured as in Example 1.

The conversion coating weight was measured as in Example 1. The coatingwas stripped in these measurements using the following procedures.

Stripping Conditions

(1) For the cold-rolled steel sheet

stripping solution: 5% aqueous chromic acid

stripping conditions: 75° C., 15 minutes, immersion stripping

(2) For the zinc-plated sheet

stripping solution: 2% by weight ammonium dichromate+49% by weight of28% aqueous ammonia+49% by weight pure water

stripping conditions: room temperature, 15 minutes, immersion stripping

(3) For the aluminum-magnesium alloy sheet

stripping solution: 5% aqueous chromic acid

stripping conditions: room temperature, 5 minutes, immersion stripping

The deposited coating crystals were inspected with a scanning electronmicroscope (“SEM”) at 1,000×. This magnified image was used to evaluatesubstrate metal coverage (presence or absence of exposed substrate) andto measure the particle size of the conversion coating crystals forevaluation of finely sized crystal formation.

The following standards were used for reporting the substrate metalcoverage and for evaluation of coating grain size.

(1) In Standard for evaluation of coating grain size

++ less than 30 micrometers (good)

+ at least 30 micrometers but less than 50 micrometers (moderately poor)

x at least 50 micrometers (poor)

(2) Standard for evaluation of substrate metal coverage

++ absolutely no exposure of substrate metal (good)

+ moderate exposure of substrate metal (moderately poor)

x substrate metal completely exposed (poor)

In Example 16, the cleaning/conversion treatment bath (6) specified below was heated to 45° C. and used to conversion treat the cold-rolledsteel sheet by immersion for 180 seconds. The resulting coating weightwas 1.2 g/m², and the coating grain size and substrate metal coveragewere both evaluated as good.

Composition of Conversion Treatment Bath (6)

phosphate ions 15 g/L (from addition of 75% phosphoric acid) zinc ions1.3 g/L (from addition of zinc oxide) nickel ions 0.5 g/L (from additionof nickel carbonate) fluorine component 1.0 g/L (from addition of sodiumfluosilicate) organoperoxide 500 ppm (of tert-butyl hydroperoxide)tert-butanol 4.0 g/L surfactant 1.0 g/L (addition of polyoxyethylene-polyoxypropylene block polymer with an average molecular weight of10,000 and an ethylene oxide addition proportion of 80%) oil component2.0 g/L (addition of NOX-RUST ™ 550) free acidity 0.6 point

The test results are reported in Table 6.

TABLE 6 Ident- Coating Cover- ifica- Sub- Grain Size age tion strateAccelerator(s) Surfactant(s) Rating Rating Ex 16 SPC a: 500 A: 1.0 ++ ++Ex 17 EG a: 500 A: 1.0 ++ ++ Ex 18 SPC f: 1000 B: 1.0 + C: 0.5 ++ ++ Ex19 EG f: 1000 B: 1.0 + C: 0.5 ++ ++ Ex 20 SPC g: 100 D: 1.5 + E: 0.5 ++++ Ex 21 EG g: 100 D: 1.5 + E: 0.5 ++ ++ Ex 22 AL g: 100 D: 1.5 + E: 0.5++ ++ CE 10 SPC d: 100 + h: 7000 None None x CE 11 EG d: 100 + h: 7000A: 1.0 x ++ CE 12 SPC None B: 1.0 + C: 0.5 x x CE 13 EG e: 1500 B: 1.0 +C: 0.5 x + CE 14 AL e: 1500 B: 1.0 + C: 0.5 None x AdditionalAbbreviation in. and Notes for, Table 6 “a” means tert-butylhydroperoxide (an organoperoxide); “f” means di-tert-butyl peroxide (anorganoperoxide); “g” means acetylacetone peroxide (an organoperoxide);“h” means nitrate ions; “d” means nitrite ions; “e” means sodiumchlorate. “A” means polyoxyethylene-polyoxypropropylene block polymer;“B” means polyoxyethylene sorbitan monolaurate; “C” means lauryl ethersulfate ester salt; “D” means polyoxyethylene oleyl ether; “E” meanslauryldimethylbetaine. In the column headed “Coating Grain Size Rating”,the entry “None” means that no coating formed.

In Example 17, the cleaning/conversion treatment bath (6) described inExample 16 was used to conversion treat the zinc-plated sheet byimmersion for 180 seconds. The resulting coating weight was 3.5 g/m²,and the coating grain size and substrate metal coverage were bothevaluated as good. The test results are reported in Table 6.

In Example 18, the cleaning/conversion treatment bath (7) specifiedbelow was heated to 40° C. and used to conversion treat the cold-rolledsteel sheet by spraying for 120 seconds. The resulting coating weightwas 1.2 g/m², and the coating grain size and substrate metal coveragewere both evaluated as good.

Composition of Conversion Treatment Bath (7)

phosphate ions 14 g/L (from addition of 75% phosphoric acid) zinc ions1.3 g/L (from addition of zinc oxide) cobalt ions 0.5 g/L (from additionof basic cobalt carbonate) organoperoxide 1000 ppm (from addition ofdi-tert-butyl peroxide) tert-butanol 2.0 g/L surfactant 1.0 g/L (fromaddition of polyoxyethylene sorbitan monolaurate with moles of EOaddition = 20) 0.5 g/L (from addition of lauryl ether sulfate ester saltwith moles of EO additon = 3) oil component 3.0 g/L (from addition ofNOX-RUST ™ 550) free acidity 0.5 point

The test results are reported in Table 6.

In Example 19, the cleaning/conversion treatment bath (7) described inExample 18 was used to conversion treat the zinc-plated sheet byspraying for 120 seconds. The resulting coating weight was 3.3 g/m², andthe coating grain size and substrate metal coverage were both evaluatedas good. The test red suits are reported in Table 6.

In Example 20, the cleaning/conversion treatment bath (8) specifiedbelow was heated to 43° C. and used to conversion treat the cold-rolledsteel sheet by spraying for 30 seconds and then immersion for 90seconds. The resulting coating weight was 1.3 g/m², and the coatinggrain size and substrate metal coverage were both evaluated as good.

Composition of Conversion Treatment Bath (8)

phosphate ions 17 g/L (from addition of 75% phosphoric acid) zinc ions1.5 g/L (from addition of zinc oxide) fluorine component 0.4 g/L (fromaddition of sodium fluoride) organoperoxide 100 ppm (from addition ofaceylacetone peroxide) oil component 2.0 g/L (from addition ofNOX-RUST ™ 550) surfactant 1.5 g/L (from addition of polyoxyethyleneoleyl ether with moles of EO addition = 7) 0.5 g/L (from addition oflauryldimethylbetaine) free acidity 0.7 point

The test results are reported in Table 6.

In Example 21, the conversion treatment bath (8) described for Example20 was used to conversion treat the zinc-plated sheet by spraying for 30seconds and then immersion for 90 seconds. The resulting coating weightwas 3.6 g/m², and the coating grain size and substrate metal coveragewere both evaluated as good.

The test results are reported in Table 6.

In Example 22, the conversion treatment bath (8) described for Example20 was used to conversion treat the aluminum alloy sheet by spraying for30 seconds and then immersion for 90 seconds. The resulting coatingweight was 2.5 g/m², and the coating grain size and substrate metalcoverage were both evaluated as good.

The test results are reported in Table 6.

In Comparative Example 10, the conversion treatment bath (9) specifiedbelow was heated to 45° C. and used to conversion treat the cold-rolledsteel sheet by immersion for 180 seconds. Because neither organoperoxidenor surfactant was added to this treatment bath, the oil component wasnot removed even upon completion of the treatment and coating depositionwas completely absent.

Composition of Conversion Treatment Bath (9)

phosphate ions 15 g/L (from addition of 75% phosphoric acid) zinc ions1.3 g/L (from dissolution of zinc oxide) nickel ions 0.5 g/L (fromaddition of nickel nitrate) fluorine component 1.0 g/L (from addition ofsodium fluosilicate) nitrate ions 7.0 g/L (from addition of nickel andsodium nitrates) nitrite ions 100 ppm (from addition of sodium nitrite)oil component 2.0 g/L (addition of NOX-RUST ™ 550) free acidity 0.6point

The test results are reported in Table 6.

In Comparative Example 11, the conversion treatment bath (10) specifiedbelow was heated to 45° C. and used to conversion treat the zinc-platedsheet by immersion for 180 seconds. The resulting coating weight was 5.3g/m², and the substrate metal coverage was evaluated as good. However,because no organoperoxide was present, the crystal particles were coarseand coating grain size was evaluated as poor.

Composition of Conversion Treatment Bath (10)

phosphate ions 15 g/L (from addition of 75% phosphoric acid) zinc ions1.3 g/L (from addition of zinc oxide) nickel ions 0.5 g/L (from additionof nickel nitrate) fluorine component 1.0 g/L (from addition of sodiumfluosilicate) nitrate ions 7.0 g/L (from addition of nickel and sodiumnitrates) nitrite ions 100 ppm (from addition of sodium nitrite)surfactant 1.0 g/L (from addition of polyoxyethylene- polyoxypropyleneblock polymer with an average molecular weight of 10,000 and an ethyleneoxide addition proportion of 80%) oil component 2.0 g/L (from additionof NOX-RUST ™ 550) free acidity 0.6 point

The test results are reported in Table 6.

In Comparative Example 12, the conversion treatment bath (11) specifiedbelow was heated to 40° C. and used to conversion treat the cold-rolledsteel sheet by spraying for 120 seconds. The resulting coating weightwas 0.3 g/m². However, there was an absence of organoperoxide, and thecoating grain size and substrate metal coverage were both evaluated aspoor.

Composition of Conversion Treatment Bath (11)

phosphate ions 14 g/L (from addition of 75% phosphoric acid) zinc ions1.3 g/L (from addition of zinc oxide) cobalt ions 0.5 g/L (from additionof basic cobalt carbonate) surfactant 1.0 g/L (from addition ofpolyoxyethylene sorbitan monolaurate with moles of EO addition = 20) 0.5g/L (from addition of lauryl ether sulfate ester salt with moles of EOadditon = 3) oil component 3.0 g/L (from addition of NOX-RUST ™ 550)free acidity 0.5 point

The test results are reported in Table 6.

In Comparative Example 13, the conversion treatment bath (12) specifiedbelow was heated to 40° C. and used to conversion treat the zinc-platedsteel sheet by spraying for 120 seconds. The resulting coating weightwas 2.1 g/m². However, there was an absence of organoperoxide, and thecoating grain size was evaluated as poor and the substrate metalcoverage was evaluated as moderately poor.

Composition of Conversion Treatment Bath (12)

phosphate ions 14 g/L (from addition of 75% phosphoric acid) zinc ions1.3 g/L (from addition of zinc oxide) cobalt ions 0.5 g/L (from additionof basic cobalt carbonate) chlorate ions 1.5 g/L (from addition ofsodium chlorate) surfactant 1.0 g/L (from addition of polyoxyethylenesorbitan monolaurate with moles of EO addition = 20) 0.5 g/L (fromaddition of lauryl ether sulfate ester salt with moles of EO additon =3) oil component 3.0 g/L (addition of NOX-RUST ™ 550) free acidity 0.5point

The test results are reported in Table 6.

In Comparative Example 14, the conversion treatment bath described forComparative Example 13 was used to conversion treat the aluminum sheetby spraying for 120 seconds. However, film deposition was entirelyabsent due to the absence of the organoperoxide.

Table 6 reports the substrates, the conversion accelerators andsurfactants in the conversion treatment baths, and the results of thepost-treatment evaluation of the coating crystals for Examples 16 to 22and Comparative Examples 10 to 14. These results confirm that Examples16 to 22, which employed a surface treatment method according to thepresent invention, were able to clean even the surface of oil-coatedmetal while simultaneously depositing thereon a uniform, fine, and densezinc phosphate-type conversion coating.

Comparative Example 10 involved treatment with a surfactant-freeconversion treatment bath, and in contrast to the above results wasunable to deposit a conversion film due to an inadequate removal of theoil/grease component Comparative Example 12 involved treatment with aconversion accelerator-free treatment bath, and in this case themicrofine-sizing of the crystals in the coating and coating coveragewere inadequate. Comparative Examples 11 and 13 concerned treatment withorganoperoxide-tree baths that contained inorganic conversionaccelerators. In these cases, the film crystals were coarse, so that auniform, fine, and dense conversion film was not obtained. ComparativeExample 14 used an inorganic conversion accelerator, but a conversionfilm was not formed.

Benefits of the Invention

Because a zinc phosphate-based conversion treatment bath according tothe present invention—and hence a bath used in a treatment methodaccording to the present invention—is substantially free of nitrogenouscompounds, effluent from the method according to the present inventionis also environmentally nonpolluting as a practical matter, and the bathand method according to the present invention are therefore able to meetenvironmental regulations and restrictions. The general limitation ofthe total nitrogenous compound content in the bath to 0 to 200 ppm asnitrogen poses very little risk of environmental pollution.

A conversion treatment bath and surface treatment method according tothe present invention cause the deposition of uniform, fine, and densezinc phosphate-type conversion films on metals. These films also supportan excellent painting performance, for example, in terms ofpost-painting corrosion resistance and water-resistant secondaryadherence. Moreover, the invention uses a very simple process sequence,i.e., cleaning (degreasing)—conversion treatment—water rinse. As aresult, the surface treatment method using a conversion bath accordingto the present invention does not require the surface-conditioningtreatment required in the prior art for the deposition of uniform, fine,and dense conversion films. As this provides a number of advantages,such as a simplification of the treatment facilities, release fromcomplicated bath management, and savings because surface conditioner isno longer required, the bath and method according to the presentinvention clearly represent a major technological advance.

Moreover, through the introduction of a surfactant for surface cleaninginto the conversion bath according to the present invention, degreasingand zinc phosphate-based conversion treatment can be simultaneouslyeffected in a single step on the surfaces of metals that bear, forexample, oil and/or grease. This also yields a uniform, fine, and denseconversion coating. The merits accruing to the use of thecleaning/conversion treatment method according to the present inventionextend over a broad range, including, for example, a substantialshortening of the treatment sequence, simplification of the treatmentfacilities, space savings, increased productivity, a reduction inreagent costs, simplification of reagent management, and so forth.

The invention claimed is:
 1. A liquid zinc phosphate conversion coatingbath composition comprising water, zinc ions, phosphate ions, and from50 to 1500 ppm of an organoperoxide conversion accelerator selected fromthe group consisting of ethyl hydroperoxide, isopropyl hydroperoxide,tert-butyl hydroperoxide, tert-hexyl hydroperoxide, diethyl peroxide,di-tert-butyl peroxide, acetylacetone peroxide, cumene hydroperoxide,tert-butylperoxymaleic acid, monoperphthalic acid, and persuccinic acid.2. A bath composition according to claim, wherein nitrogenous compoundsare present, if at all, only in an amount having a stoichiometricequivalent as nitrogen of not more than 200 ppm.
 3. A bath compositionaccording to claim 2, comprising from 50 to 1500 ppm of organoperoxidesthat are water-soluble and have a peroxy structure or a percarboxylstructure.
 4. A bath composition according to claim 3, wherein saidorganoperoxides are selected from ethyl hydroperoxide, isopropylhydroperoxide, tert-butyl hydroperoxide, tert-hexyl hydroperoxide,diethyl peroxide, di-tert-butyl peroxide, acetylacetone peroxide, cumenehydroperoxide, tert-butylperoxymaleic acid, peracetic acid,monoperphthalic acid, and persuccinic acid.
 5. A bath compositionaccording to claim 4, which also contains from 0.5 to 5.0 g/L ofsurfactant.
 6. A bath composition according to claim 3, which alsocontains from 0.5 to 5.0 g/L of surfactant.
 7. A bath compositionaccording to claim 1, which also contains a surfactant.
 8. A bathcomposition according to claim 7, having a pH value from 2 to
 4. 9. Aprocess of forming a zinc phosphate conversion coating on a metalsubstrate, said process comprising a step of contacting the metalsubstrate with a composition according to claim 8 at a temperature of25° C. to 50° C.
 10. A process of forming a zinc phosphate conversioncoating on a metal substrate, said process comprising a step ofcontacting the metal substrate with a composition according to claim 7at a temperature of 25° C. to 50° C.
 11. A bath composition according toclaim 5, having a pH value from 2 to
 4. 12. A bath composition accordingto claim 4, having a pH value from 2 to
 4. 13. A process of forming azinc phosphate conversion coating on a metal substrate, said processcomprising a step of contacting the metal substrate with a compositionaccording to claim 12 at a temperature of 25° C. to 50° C.
 14. A processof forming a zinc phosphate conversion coating on a metal substrate,said process comprising a step of contacting the metal substrate with acomposition according to claim 11 at a temperature of 25° C. to 50° C.15. A process of forming a zinc phosphate conversion coating on a metalsubstrate, said process comprising a step of contacting the metalsubstrate with a composition according to claim 6 at a temperature of25° C. to 50° C.
 16. A process of forming a zinc phosphate conversioncoating on a metal substrate, said process comprising a step ofcontacting the metal substrate with a composition according to claim 5at a temperature of 25° C. to 50° C.
 17. A process of forming a zincphosphate conversion coating on a metal substrate, said processcomprising a step of contacting the metal substrate with a compositionaccording to claim 4 at a temperature of 25° C. to 50° C.
 18. A processof forming a zinc phosphate conversion coating on a metal substrate,said process comprising a step of contacting the metal substrate with acomposition according to claim 3 a temperature of 25° C. to 50° C.
 19. Aprocess of forming a zinc phosphate conversion coating on a metalsubstrate, said process comprising a step of contacting the metalsubstrate with a composition according to claim 2 at a temperature of25° C. to 50° C.
 20. A process of forming a zinc phosphate conversioncoating on a metal substrate, said process comprising a step ofcontacting the metal substrate with a composition according to claim 1.